Not as useful as we like to think.

Although certainly better than traditional forms like xinfang/shangfang or the traditional media, reporting corruption of Weibo is also subject to a number of constraints. The real-name system recently (well, not that recently) implemented is an important impediment as it is possible to tell who reported the officials - they are then subject to menacing by police officers or even threats from the government officials themselves (Zhu Ruifeng got death threats; Wang Peirong was beaten up and sent to jail). Plus, reporting is, well, just reporting - many local governments do not have clean judiciaries and it is hard to actually prosecute the official. I don't deny that Weibo reporting has had some success (Zhu Ruifeng was really good at it), but its utility is still very limited compared to, say, reporting to the ICAC in Hong Kong.